Gas chromatograph mass spectrometers (GC/MS) are nowadays widely used in qualitative as well as quantitative analyses of various sample substances. A conventional gas chromatograph mass spectrometer is described referring to FIG. 3. The Unexamined Japanese Patent Publication Nos. H10(1998)-283982 and 2001-208740 disclose such gas chromatograph mass spectrometers.
In a gas chromatograph (GC) part 10, a sample atomizer 11 is provided at the entrance of a capillary column 14, which is enclosed by a column oven 13. A carrier gas flows through the sample atomizer 11 and the column 14 at a constant rate. When a liquid sample is injected by an injector 12 into the sample atomizer 11, the sample is instantaneously atomized and is carried by the carrier gas to the column 14. The column 14 is heated by the column oven 13 at a predetermined temperature so that components contained in the sample are separated with respect to time while the sample passes through the column 14. The sample gas, including the separated components, is introduced via an interface 20 to an ionizing chamber 31 of a mass spectrometer (MS) part 30. The mass spectrometer part 30 is housed in a vacuum chamber 35 which is evacuated by a pump. The sample molecules are ionized in the ionizing chamber 31, where various ionizing methods can be used including the electron impact (EI) ionization method. Ions thus generated are drawn out of the ionizing chamber 31, converged by an ion lens 32, and introduced to a quadrupole mass filter 33. A combination of a DC voltage and an AC voltage is applied to the quadrupole mass filter 33, and ions having a specific mass to charge ratio corresponding to the applied voltage can pass the quadrupole mass filter 33, and are detected by an ion detector 34.
The primary purpose of the interface 20 connecting the GC part 10 and the MS part 30 is to maintain the temperature at about the exit of the column 14 at almost the same as that inside the column oven 13, whereby the sample gas is constantly introduced into the ionizing chamber 31 without disruption. The interface 20 thus contains a heater unit.
In the GC part 10 of a GC/MS, various columns having different selectivity characteristics are used depending on the object and kind of the sample to be analyzed. Thus an exchange of columns is often necessary, which is one of troublesome operations of a GC/MS.
In a type of GC/MS with sample atomizers 11a, 11b, two (or more) columns 14a, 14b are provided in the column oven 13 as shown in FIG. 4, and an appropriate one of the two columns 14a, 14b is used according to the object of the analysis and the sample. In such a type of GC/MS, the exits of the two columns 14a, 14b are merged with a T-joint 15, and the merged column 16 is extended to the ionizing chamber 31 via the interface 20. In such a type, however, the gas pressure in the T-joint 15 is influenced by those in the two columns 14a and 14b, so that the calculation of the carrier gas flow, which is determined by the pressures at the entrance and at the exit of a column, becomes complicated and less precise.
The above problem can be solved by extending the two columns 14a and 14b in parallel to the ionizing chamber 31 via the interface 20, rather than joining them before the interface 20. Since, in this method, the exits of the two columns 14a and 14b are in a vacuum, the flow of the carrier gas can be calculated precisely as in the case of a single column.
The above construction has a drawback as follows. As shown in FIG. 5, the interface 20 includes a tubular heat-retention metal block 21 whose end is made into a threaded bolt 211. A metal nut 22 is screwed into the bolt 211 with a plastic ferrule 23 between them. The column 14 is inserted in a small central hole 231 of the plastic ferrule 23. As the nut 22 is screwed into the bolt 211 of the heat-retention block 21 to a certain strength, the plastic ferrule 23 is pressed to the end of the bolt 211 and its central hole 231 tightly holds the column 14. Since the central hole 212 of the heat-retention block 21 opens to the ionizing chamber 31 as shown in FIG. 3, the inside of the central hole 212 is also in a vacuum. Since the ferrule 23 seals around the column 14, gas is prevented from leaking from the GC part 10 into the MS part 30.
In the case of two columns 14a and 14b extending in parallel to the ionizing chamber 31, the construction of the interface 20 shown in FIG. 5 becomes as shown in FIGS. 6A and 6B. Since the two columns 14a and 14b should be separated by a certain distance in the column oven 13, a ferrule 23a having two small holes 23a1 separated by the distance, as shown in FIGS. 6A and 6B, is used. Correspondingly, the nut 22a must have a central opening 22a1 larger than the outer distance A of the two holes 23a1 of the ferrule 23a, and the heat-retention block 21a must have a central opening 21a2 larger than the outer distance A.
Using such an interface 20, it is possible to use two columns 14a and 14b in the gas chromatograph part 10 of the GC/MS. But, in many cases, only a single column is used with such a two-column interface. In this case, the one-column nut 22b and one-hole ferrule 23b as shown in FIG. 6C are used instead of those shown in FIG. 6A.
There is a problem in this case. Since the central opening 21a2 of the heat-retention block 21a is large, the contact area B between an end of the heat-retention block 21 and that of the ferrule 23b is rather small as shown in FIG. 6D, which weakens the air seal effect. The problem can be avoided by using the proper heat-retention block 21 for a single column, or by changing the whole interface to the one-column interface 20 as shown in FIG. 5. But the changing operation of the interface 20 needs a lot of care and is time-consuming. Preparing two sets of interfaces is also financially disadvantageous.